This invention relates to electrostatographic copying and more particularly to a novel method for imaging a particular type of electrostatographic photoreceptor. The art of xerography, as originally disclosed in U.S. Pat. No. 2,297,691 by C. F. Carlson, involves the formation of an electrostatic latent image on the surface of a photosensitive plate normally referred to as a photoreceptor. The photoreceptor comprises a conductive substrate having on its surface a layer of a photoconductive insulating material. Normally, there is a thin barrier layer between the substrate and the photoconductive layer to prevent charge injection of the minority carrier from the substrate into the photoconductive layer upon charging of the plate's surface.
In operation, the plate is charged in the dark, such as by exposing it to a cloud of corona ions, and imaged by exposing it to a light and shadow image to selectively discharge the photoreceptor and leave a latent image corresponding to the shadow areas. The latent electrostatic image is developed by contacting the photoreceptor's surface with an electroscopic marking material known as toner which will adhere to the latent image due to electrostatic attraction. Transfer of the toner image to a receiving member such as paper with subsequent fusing of the toner into the paper provides a permanent copy.
One type of electrostatographic photoreceptor comprises a conductive substrate having a layer of photoconductive material on its surface which is overcoated with a layer of an insulating organic resin. Various methods of imaging this type of photoreceptor are disclosed by Mark in his article appearing in Photographic Science and Engineering, Vol. 18, No. 3, May/June 1974. The processes disclosed in this article all employ a separate presensitization step which involves exposing the photosensitive device to d.c. corona of a polarity opposite to that of the majority charge carrier. When applying a positive charge to the surface of the insulating layer as in the case where an n-type photoconductor is employed, a negative charge is induced in the conductive substrate, injected into the photoconductor and transported to and trapped at the insulating layer-photoconductive layer interface resulting in an initial potential being solely across the insulating layer. The charged plate is then exposed to a light and shadow pattern either during or after the application of a secondary electrostatic charge which may be either a.c. current or d.c. current of polarity opposite to that of the initial charge. Flooding uniformly with light reduces the potential in the photoconductive layer to zero volts and results in the formation of a latent image on the surface of the device which is developed in the normal xerographic mode by the application toner thereto. After transfer of the toner to a transfer member, the latent image is erased by the simultaneous application of light and a d.c. or a.c. corona device and the device is prepared for the next cycle by again presensitizing it.
The present invention condenses the erasure and presensitization steps into a unitary operation thereby reducing the usual four step process (initial charge, shuntexpose, flood, erase) to a three step process.